Quantum cascade lasers (QCLs) are unipolar semiconductor lasers that use optical transitions between electronic sub-bands to produce light. QCLs can be designed to emit in the mid-infrared wavelength range (e.g., 2 μm-20 μm), and more recently in the long-infrared wavelength range, e.g., the terahertz spectral range. QCL technology has generally reached a maturity level where it can provide relatively reliable operation for use in a large variety of applications. By way of example and not limitation, detectors that incorporate QCLs can be used for chemical sensing such as pollution monitoring, gas sensing, medical diagnostics (e.g., through breath analysis), the remote detection of toxic chemicals and explosives, and others. For applications requiring radiation at a single frequency, the longitudinal mode selection in QCLs may be provided, where single longitudinal mode operation of QCLs can be achieved by fabricating the QCLs as distributed feedback lasers (DFB-QCL).
In a typical DFB-QCL device, a grating has grooves etched at the top of the device that are aligned perpendicular to the optical axis of the device to produce index coupling, which selects the longitudinal mode for single frequency emission. Many embodiments of this basic idea exist to improve the selection of the single longitudinal mode in such DFB-QCL devices.
In contrast to the longitudinal mode, the transverse (or lateral) mode in QCLs is generally not selectively controlled in existing devices. Instead, a QCL device may have a sufficiently narrow width such that only the fundamental transverse mode is active. The fundamental transverse mode ensures that a single diffraction-limited beam along the optical axis of the laser is emitted, with an angular divergence determined by the wavelength of the light and the width of the device. The power level that can be generated in a QCL, as in other semiconductor lasers, may scale with the area of the device. In instances where additional power is desired, a larger area QCL device may be fabricated, where a QCL device with a cavity width larger than 12-15 micrometers is generally referred to as a broad area device (BA-QCL).
In practice, scaling of the power by enlarging the area of the QCL is typically limited to increasing the cavity length, rather than the cavity width of the QCL. This is because keeping a narrow cavity can maintain fundamental transverse mode operation, whereas increasing the cavity width may result in the operation of high-order transverse modes, as they become more favorable. For example, if an existing mid-infrared QCL is fabricated with a cavity width of about 15 micrometers, this may lead to the emergence of high-order transverse modes, resulting in mode competition, beam steering, and loss of brightness. In cavity widths of about 20 micrometers and higher, one or several high-order transverse modes may be active where each high-order transverse mode forms a periodic structure in the near-field of the QCL device, resulting in a laser beam with two distinct lobes, where each lobe deviates from the optical axis by an angle that becomes larger as the mode number increases—see, e.g., Y. Bai et al., APPLIED PHYSICS LETTERS 95, 221104 (2009), which is hereby incorporated by reference.
Several approaches have been attempted to produce single lobed emission—the result of a fundamental transverse mode—in BA-QCLs. These include the use of angled cavities, photonic crystal gratings, gain-guided devices, and the use of a porous structure above the active region of the device. However, there are some disadvantages associated with these techniques. For example, in angled cavity configuration, the facet angles and the cavity length must be precisely controlled for single lobed emission—see, e.g., D. Heydari et al., APPLIED PHYSICS LETTERS 106, 091105 (2015), which is hereby incorporated by reference. In gain-guided devices, the current spreading determines the effective width of the device, and this results in a strong variation of the beam divergence with injection current—see, e.g., I. Sergachev et al., OPTICS EXPRESS 24, 19063 (2016), which is hereby incorporated by reference. In another approach, lateral constrictions in the waveguide were placed using a focused ion beam milling technique where only the fundamental mode was allowed to propagate to produce a Gaussian shaped far-field pattern—see, e.g., Bouzi et al., APPL. PHYS. LETT. 102, 122105 (2013), which is hereby incorporated by reference. However, this approach was limited to devices with a narrow cavity width (w=10 μm), not BA-QCLs, and the trenches had to be filled with metal to provide additional losses to achieve the desired effect. Therefore, there remains a need for improved devices, systems, and methods for extracting and maintaining fundamental transverse mode operation in BA-QCLs.